There is an optical power monitor in which a part of an optical signal propagating in an optical fiber is branched to detect its strength. Optical power monitors are disclosed in Patent Documents 1, 2 and 3. Since the optical power monitors disclosed have the same basic configuration, the illustration in FIG. 3 of Patent Document 1 is shown in FIG. 8, and the structure will be described with reference to FIG. 8.
An optical power monitor 800 shown in FIG. 8 includes an optical fiber 812 on an incoming side, an optical fiber 813 on an outgoing side, a two-core capillary 810 (corresponding to “pig-tail fiber” in the present invention), a cylindrical gradient index lens 820 having the same diameter as the two-core capillary 810 (corresponding to “Graded Index Lens”, i.e. “GRIN lens” in the present invention), a dot mirror 830 (corresponding to “tap film” in the present invention), a photodetector 840 (corresponding to “photo-diode” in the present invention), an optical receiver circuit 844, and an indicator 846.
A light signal input to the optical fiber 812 on the incoming side passes through the two-core capillary 810 and is focused onto the dot mirror 830 by the gradient index lens 820. Most of the focused light is reflected by a surface of the dot mirror 830, goes back to the gradient index lens 820, and output through the two-core capillary 810 by the optical fiber 813 on the outgoing side. Light passing through the dot mirror 830 is detected by the photodetector 840, and the light signal is converted into an electric signal. The electric signal passes through the optical receiver circuit 844 and is indicated as the strength of light by the indicator 846. Unless otherwise specially specified herein, “energy of light” and “strength of light” may be used as synonyms.
The two-core capillary, i.e. a pig-tail fiber or a capillary glass ferrule, and the gradient index lens, i.e. a GRIN lens are positioned opposed to and at a predetermined distance from each other. A pig-tail fiber and a GRIN lens shown in Patent Documents 1, 5 and 6 have their opposite end faces forming an angle of about 8° with an axis (optical axis), and in Patent Documents 2 to 4, a pig-tail fiber and a GRIN lens having their opposite end faces perpendicular to an axis are disclosed. In order that the end faces perpendicular to the axes of the pig-tail fiber and the GRIN lens as shown in Patent Documents 2 to 4 are positioned to be opposed to each other at the predetermined distance therebetween, the opposite end faces are adjusted to be parallel to each other, providing easy assembling. However, a part of the light is reflected by the end faces of the pig-tail fiber and the GRIN lens and goes back in the core, causing interference with light progressing later to generate reflection loss of light. In order to reduce the reflection loss, as disclosed in Patent Documents 5 and 6, the opposite end faces are angled by an angle of about 6° to about 10°, an average angle of 8°. The reflection loss can be reduced to 50 to 55 (dB) by angling the opposite end faces relative to the axis. However, it becomes very difficult to assemble the pig-tail fiber and the GRIN lens.
Patent Document 1: Japanese Patent Laid-Open No. 2003-202262
Patent Document 2: Japanese Patent Laid-Open No. 62-269909
Patent Document 3: U.S. Pat. No. 6,603,906
Patent Document 4: U.S. Pat. No. 5,790,314
Patent Document 5: Japanese Patent Laid-Open No. 2001-013362
Patent Document 6: Japanese Publication of International Patent Application No. 10-511476